(1) Field of the Invention
The present invention relates to a process for plating palladium on niobium, vanadium, zirconium, tantalum and titanium. In particular the present invention provides a very adherent plate of palladium on these metals in pure form or as alloys.
(2) Prior Art
Metal membrane hydrogen extractors have two main applications as developed for the nuclear and petroleum industries: economical hydrogen recovery and shifting the thermodynamics of otherwise unfavorable reactions. The main advantages of metallic membranes over polymers is that they generally accept higher temperatures and more corrosive environments. Also, the pressure drop for hydrogen recovery with metallic membranes can be much smaller than with polymeric membranes (as low as 10.sup.-8 torr for hydrogen recovery from liquid lithium) and the hydrogen is recovered at essentially 100% purity. These benefits can offset the generally higher fixed cost of metallic membranes.
Palladium and palladium alloys are the historic choice for metallic hydrogen extraction membranes. Their cost is high because of palladium's high cost-per-pound and low strength, but no other single metal combines palladium's high permeability and good surface properties. For many purposes, low cost composite membranes using palladium coated refractory metals appear to be as good or better than palladium. For example, a palladium coated zirconium or titanium membrane can be used for hydrogen extraction from a nuclear heat transfer fluid, for extraction and economical recovery of the hydrogen isotope.
In the past, gas sputtering has been used to coat palladium on various metals. Gas sputtering is inferior to electroless or electrolytic plating in providing a coating inside tubes, a preferred geometry for use for the purpose of hydrogen extraction. Until now, the only successful method reported for direct electroless or electrolytic plating of palladium on the refractory metals was a technique, reported below, for coating on zirconium.
U.S. Pat. No. 3,350,844 to Makrides et al discloses hydrogen separation using palladium on a group V-B metal (niobium, vanadium and tantalum). The palladium coating is accomplished by sputtering. Boes, N., et al, Z. Naturforsch. 31 A, 754-759 (1976) describes vapor deposition of palladium on vanadium or niobium. Pick, M. A., The Kinetics of Hydrogen Adsorption-Desorption by Metals, G. Bombakidis ed. Proceeding of NATO Advanced Study Inst. on Metal Hydrides, June 17-27, Rhodes, Greece (Plenum Press) pp. 329-343 (1981) also describes vapor deposition of palladium on niobium. Hsu and Buxbaum, J. Electrochemical Soc. 132 2419-2420 (1985) describes plating of palladium on zirconium using electroless plating. A zirconium hydride, deposited by chemical action to enhance the electroless plating, is described. Chemical hydriding produced poorly adherent coatings when applied to refractory substrates besides zirconium (i.e. Nb and V).